A top drive assembly is used in oilfield operations for manipulating a wellbore string such as a drill string or a casing or liner string. The top drive assembly is supported in a rig such as a mast or derrick for supporting and axially and rotationally driving the wellbore string.
Referring to FIG. 1, an example of a prior art top drive assembly is shown. The top drive assembly 10 may be supported from a hook in a rig (not shown) by use of a hoisting apparatus 12 such as a bail, links, blocks, etc., and may be stabilized by connection to a torque track 14. A top drive assembly generally may include a quill 16 that extends from the top drive assembly downwardly for connection directly, or indirectly as through a clamp 18 etc., to the wellbore string 20, a swivel 22 including a swivel bearing for rotationally supporting the quill, a torque and drive system 24 that may include, for example, a gear box 26 and motor 28 for applying torque to the quill, and link arms 30 with elevators 34 connected via link hangers 31 on a collar 32 and load nut 33 to the quill for supporting the weight of the wellbore string when the wellbore string is supported by the top drive assembly through elevators 34 but not connected to the quill.
In a typical configuration, as shown, swivel 22 is positioned adjacent the upper end of the top drive assembly with bail 12 connected to the swivel housing. Quill 16 is supported in the swivel and extends down through drive system 24 to extend out at the bottom of assembly 10. A typical top drive assembly may accommodate a space of about 20 ft (6 m) or more above from the top of the bail to the top of the link arm elevators.
In operations employing a top drive assembly; a load path is therefore set up from the wellbore string through the quill to the swivel and then to the bail through which the load is conveyed to the rig hoisting equipment. When the wellbore string is supported on the elevators, the load path extends through elevators 34 and link arms 30 to the load collar and the load nut and then up through the quill to swivel 22 and bail 12. A typical top drive assembly, therefore, effectively forms a single load path from top to bottom, which is through the quill.
Although the top drive assembly is an integral part of modern wellsite operations, the typical top drive configuration has presented problems with size and reliance on the quill in the load path.